Vital work has been done on the so-called "garden room," 16'x16' glass-fronted room, with windows raised within the roof. On its parkland side, the couple will eventually erect a pergola, to allow enable the growth of vines or something similar to provide shade and privacy.

The new room spans a gap between the main house and some old wood stores, which are to be converted into rooms, including kitchen and lounge and bedroom. It has been suggested that this will be for nanny Maria Borrallo.

Already, the vast Georgian mansion has an outdoor swimming pool and tennis court, and is surrounded by acres of parkland of the Queen's Sandringham estate – just perfect for raising children away from the hubbub of William and Kate's Apartment 1a at Kensington Palace.

"It's like a fancy farmhouse," someone familiar with the house tells PEOPLE. "It isn't grandiose. But it's a great, friendly family environment."